Gas-phase olefin polymerization processes are economical processes for the polymerization of olefins. Such gas-phase polymerization processes can, in particular, be carried out in gas-phase fluidized-bed reactors in which the polymer particles are kept suspended by means of an appropriate gas stream. Processes of this type are described, for example, in European patent applications EP-A-0 475 603, EP-A-0 089 691 and EP-A-0 571 826, whose contents are hereby fully incorporated by reference.
In the production of polyolefins, in order to produce different polymer grades in the same reactor, there is the need, from time to time, to change the catalyst system. Therefore with a certain frequency, depending on the flexibility required to the reactor and on the production plans, it is necessary to use a first catalyst system to produce a first polymer and, subsequently, to use a second catalyst system to produce a second polymer. This change may not involve any substantial issue when a first catalyst system and the second catalyst system are compatible with one another, i.e. when both catalyst systems can operate under substantially the same process conditions (generally temperature, pressure, concentration of process auxiliaries, molar mass regulator, etc.) without substantially losing activity.
However, the change from a first catalyst system to a second catalyst system which is incompatible with the first catalyst system involves problems in ensuring an adequate continuity of production in terms of both quantity and quality of the product and has therefore been the subject of much effort.
In the present description and in the following claims, two catalyst systems are incompatible to each other if they respond in different ways to process conditions and/or monomers or any agents employed in the process (process auxiliaries), such as for example molecular weight regulators (for example hydrogen), comonomers or antistatic agents or any other agents, and if, due this different responsiveness, the polymers obtained by transitioning from the first catalyst system to the second catalyst system have unacceptable properties (e.g. molecular weight and/or melt flow rate and/or melt flow ratio out of the respective target value, presence of gels and fines, insufficient environmental crack resistance) or the process productivity is unacceptably low (e.g. due to chunks or sheeting in the reactor).
This definition applies to any of the components making part of the catalyst systems as defined above. So, in the present description and in the following claims, two catalyst systems are incompatible to each other if at least one component of the first catalyst system is incompatible with at least one component of the second catalyst system.
For example, a Ziegler-Natta catalyst system may be not compatible with a Phillips catalyst system because Ziegler-Natta catalyst systems generally require operating at antistatic agent concentrations (by way of illustrative example, preferably from 10 to 600 ppm) which kill the Phillips catalyst system.
A Ziegler-Natta catalyst system may be not compatible with a Phillips catalyst system because Ziegler-Natta catalyst systems generally require operating at a higher hydrogen concentration with respect to Phillips catalysts.
In order to perform a change between two incompatible catalyst systems, the most common method of the state of the art is that of stopping the first polymerization reaction by means of a deactivating agent, emptying the reactor, cleaning it and starting it up again by introducing the second catalyst system. Thus, for example, WO 00/58377 discloses a discontinuous method for changing between two incompatible catalysts, in which the first polymerization reaction is stopped, the polymer is removed from the reactor, the reactor is rapidly purged with nitrogen, a new seedbed of polymer particulates is introduced into the reactor and the second polymerization reaction is then started. However, on the one side the opening of the reactor leads to deposits on the walls which have an adverse effect on the renewed start-up of the reactor and, on the other side, such method inevitably requires a discontinuation of the polymerization process and an unacceptably long stop time between the first polymerization reaction and the second polymerization reaction.
It is also known from document WO 2006/069204 to transition from a Ziegler-Natta catalyst system to a Phillips catalyst system in a fluidized-bed reactor by adding to the reactor firstly a deactivating agent and then a cocatalyst adsorbing agent, for example silica, in order to absorb the metal alkyl before introducing the Phillips catalyst system.
Even in view of the teaching of the prior art, there is still the need to obtain an effective transitioning from Ziegler-Natta to Phillips catalyst systems.